The present invention relates to an optical transmitter for optical fiber communications.
With the advent of low-loss optical fibers, singlemode semiconductor lasers, such as distributed feedback semiconductor lasers (hereinafter referred to as DFB LDs for brevity), high sensitivity light receiving elements, such as germanium or InGaAs avalanche photodiodes (hereinafter referred to as APDs for brevity), and the like, optical fiber communications over a distance of more than 100 km has recently been brought to a practicable stage. Studies are presently under way for further increasing the transmission range and the transmission rate.
One of major obstructions to longer range and higher rate lightwave communications is the wavelength dispersion of optical fibers. Specifically, while in the wavelength range of 1.5-1.6 .mu.m, the transmission loss of optical fibers is minimal and long-distance transmission is practicable. The wavelength dispersion due mainly to material dispersion amounts to about -2 ps/km/.ANG. in that specific wavelength range. Hence, in order that transmission may be effected over a distance of the order of 100 km with a modulation rate of 2 Gb/s, a light source which is naturally operable in a single longitudinal mode is essential and, yet, the spread of its spectrum width has to be restricted to about 0.8 .ANG.. However, where a prior art DFB LD is directly modulated at a high rate, the carrier density of an active layer in the DFB LD is modulated and, therefore, the lasing wavelength is necessarily modulated ("wavelength chirping" as generally referred to). This causes an undesirable expansion of the spectrum line width of about 1-2 .ANG.. As such, where high-speed transmission is desired in the wavelength band of 1.5 .mu.m, the transmissible distance is limited by wavelength dispersion rather than by transmission loss.